This invention relates to the comminution of solid materials and, more particularly, is concerned with the comminution of solid materials comprising calcium carbonate.
It is known to form an aqueous suspension of a partially comminuted calcium carbonate mineral containing a dispersing agent and then to complete comminution of the calcium carbonate mineral in the aqueous suspension at a high solids content (i.e. at a solids content ranging from about 65 to about 80% by weight of solids). Thereafter the calcium carbonate mineral is either thermally dried, for example in a spray drier, or stored or transported as an aqueous suspension, i.e. in slurry form.
The energy which must be dissipated in an aqueous suspension of a calcium carbonate mineral in order to grind the mineral to a given degree of fineness (say, until 70% by weight consists of particles have an equivalent spherical diameter smaller than 2 .mu.m) is very much less when the grinding is performed using a suspension having a solids content of the order of 30% by weight than when the suspension has a solids content of the order of 70% by weight, i.e. better utilization of energy is obtained when the grinding is performed at a low solids content.
It is also known to comminute a calcium carbonate mineral in the form of a low solids content aqueous suspension (i.e. at a solids content up to about 40% by weight of solids) in the absence of a dispersing agent. Thereafter the aqueous suspension is dewatered (for example, by centrifugal or gravitational sedimentation and/or by filtration) to produce a cake of flocculated material having a solids content ranging from about 65 to about 80% by weight. This cake may be thermally dried to yield a lump or powder product or it may be mixed with a dispersing agent to deflocculate the solid and form a slurry which may be stored or transported in suitable containers. Unfortunately, filter cakes of calcium carbonate minerals which have been ground at a low solids content in the absence of a dispersing agent have proved to be very difficult, if not impossible, to deflocculate fully on completion of the comminution process.
British Patent Specification No. 1,472,701 describes a method of producing an easily pumpable aqueous suspension of a natural calcium carbonate mineral which contains at least 60% by weight of particles smaller than 2 microns equivalent spherical diameter, which method comprises the steps of (a) forming a mixture of water, a natural calcium carbonate mineral containing at least 60% by weight of particles smaller than 2 microns equivalent spherical diameter, and a small amount of calcium hydroxide, said small amount being at least 0.1% by weight based on the dry weight of the natural calcium carbonate mineral; and thereafter (b) deflocculating the natural calcium carbonate mineral with a dispersing agent to produce the desired easily pumpable aqueous suspension. The mixture of water, natural calcium carbonate mineral containing at least 60% by weight of particles smaller than 2 microns equivalent spherical diameter and calcium hydroxide formed in step (a) of this method can be obtained by agitating an aqueous suspension containing from 10% to 60% by weight of a calcium carbonate mineral, calcium hydroxide and a particulate grinding medium. Although the method described in British Patent Specification No. 1,472,701 has been found to work very well when the natural calcium carbonate mineral is chalk, i.e. a calcium carbonate mineral which was formed during the cretaceous period of geology from the remains of coccoliths or other minute organisms, it has been found that, when the calcium carbonate mineral is a harder mineral, such as marble or limestone, it is still often very difficult to deflocculate the calcium carbonate mineral in the aqueous suspension which is formed in step (a) of the method in order to form a suspension which is fluid and rheologically stable at high solids content.